A class of electrochemical sensors known as polarographic sensors has become a popular approach to the measurement of gases in a sample medium, for example measuring partial pressures of oxygen, and the like in blood, or monitoring constituent gases being applied to the patient via an anesthesia circuit in the operating theatre. In accordance with these sensors, a cathode and an anode in contact with an electrolyte are arranged so that gases migrate across a selectively permeable membrane from the sample medium into the electrolyte of the cell. Potentials are set up whereby an electrode becomes polarized and thence is depolarized by ingress of the gas, enabling current to flow in the cell. Typically, the magnitude of the current flow is proportional to the partial pressure of the reactive gas, and thus the current flowing in the cell is a representation of the gas content.
Since the early description of such cells in U.S. Pat. No. 2,913,386 to L. C. Clark, it has been well understood by those of ordinary skill in the art that application of polarographic sensors to high precision applications, such as in the health care field, entails rigorous control of the physical configuration and dimensions of the components. For example, one chief drawback was found to be the difficulty in establishing and maintaining a stable, close spacing between the active surfaces of the electrodes and the inner surface of the selectively permeable membrane. The stability of the spacing determines the stability of the diffusion gradient of the gas through the membrane and electrolyte during sensor operation, and therefore in turn determines the proportional signal level for the gas partial pressure. Response time is also thereby affected. Likewise, it was determined early on that a fixed mechanical relationship between the anode and cathode are matters of considerable importance.
Hence, in the aggregate, polarographic sensors constitute a class of apparatus which is useful and valuable in fields including health care and critical care monitoring, but which generally has entailed rigorous maintenance of tolerance and configurational limitations. Also such sensors generally are difficult to fabricate on any high production basis.
U.S. Pat. No. 3,826,730 to H. Watanabe et al., entitled "DISPOSABLE ELECTROCHEMICAL ELECTRODE" and owned by the assignee hereof, represents a substantial step forward in the field of polarographic sensors. In accordance with the Watanabe et al. patent, an electrochemical sensor comprises a permanent electrode assembly and a replaceable or disposable electrode assembly, which can be easily attached together to form an electrochemical cell. The disposable assembly includes a selectively permeable membrane rigidly attached to the housing of the disposable portion, an electrolyte well or chamber also defined by the housing and communicating with the membrane, and a metallic button or wire which is affixed to the housing against the membrane to define the cathode electrode. Typically, the cathode button or wire is a noble metal such as silver, platinum, or gold. In accordance with the Watanabe et al. teachings, the sensors also include a reusable portion, including an anode electrode for insertion into the electrolyte well, a permanent cathode for connection in circuit relationship with the cathode button or wire, and suitable external cable connections.
The Watanabe et al. design, then, while proving relatively advantageous and economical relative to its predecessors, nevertheless involves some drawbacks. Principally, these relate to relative ease or difficulty of fabrication, and to the inclusion of valuable noble metal parts in the disposable portion of the device. That is, Watanabe et al. fix both the membrane and the cathode electrode in the disposable housing and in contact with one another, and provide an anode electrolyte well in the housing in predetermined relationship to the cathode and the membrane.
It is a primary object of the present invention to provide a partially reusable, partially disposable blood gas sensor which successfully utilizes polarographic principles, yet which has a relatively simple disposable structure, and which avoids the need to discard valuable noble metal components with the disposable portion. It is, accordingly, an associated object of the present invention to provide a partially reusable, partially disposable sensor wherein the reusable portion involves rigidly mounted anode and cathode assemblies, wherein the disposable portion includes membrane and electrolyte aspects, and wherein the joining of the reusable and disposable portions automatically meets all relevant tolerances with respect to electrode-membrane-electrolyte placements.